1. Field of the Invention
The present invention relates to a liquid crystal display device having a reflective electrode on a surface of which wrinkle-like fine roughness is provided and a method of manufacturing the same.
2. Description of the Prior Art
As the liquid crystal display device, there are the transmissive liquid crystal display device for displaying the image by controlling a quantity of transmitted light every picture element and the reflective liquid crystal display device for displaying the image by controlling a quantity of reflected light every picture element. The transmissive liquid crystal display device needs a dedicated light source called a backlight, while the reflective liquid crystal display device employs the surrounding light (the sunlight or the electric lamp light) as the light source. Therefore, the reflective liquid crystal display device possesses the advantages of making possible further reduction in thickness, weight, and power consumption and also lessening eye fatigue in the long-time use over the transmissive liquid crystal display device. The reflective liquid crystal display device makes it possible to implement a display such as a paper that is impossible for the transmissive liquid crystal display device to use.
In general, the reflective liquid crystal display device has such a configuration that the TN (Twisted nematic) liquid crystal is sealed in the panel being constructed by a pair of substrates and also the polarization plate is arranged on one surface side of the panel (For example, Patent Application Publication (KOKAI) Sho 55-48733 (Patent Literature 1), Patent Application Publication (KOKAI) Hei 2-236523 (Patent Literature 2), and Patent Application Publication (KOKAI) Hei: 6-167708 (Patent Literature 3)).
FIG. 1 is a schematic sectional view showing the reflective liquid crystal display device of this type. A TN liquid crystal 13 is sealed between a first substrate 11 and a second substrate 14. A transparent plate such as a glass plate is employed at least as the substrate on the display surface side (the second substrate 14). A reflective electrode 12 made of metal or the like is formed on a surface of the first substrate 11 on the liquid crystal 13 side. A transparent electrode 15 made of ITO (Indium-Tin Oxide) or the like is formed on a surface of the second substrate 14 on the liquid crystal 13 side. An alignment film (not shown) for deciding the alignment direction of the liquid crystal molecules when no voltage is applied is formed on surfaces of the reflective electrode 12 and the transparent electrode 15 respectively. A phase plate 16 is arranged on the second substrate 14, and a polarization plate 17 is arranged on the phase plate 16.
FIG. 2A is a schematic view showing the alignment direction of liquid crystal molecules when a voltage is not applied between electrodes in the liquid crystal display device shown in FIG. 1, and FIG. 2B is a schematic view showing the alignment direction of the liquid crystal molecules when the voltage is applied between the electrodes. Although the liquid crystal molecules 13a are aligned between the electrodes 12, 15 while twisting helically in practical use, this twist of the liquid crystal molecules 13a is ignored from FIGS. 2A and 2B.
As shown in FIG. 2A, the liquid crystal molecules 13a are aligned in parallel with the substrates 11, 14 when no voltage is applied between the electrodes 12, 15. Then, liquid crystal molecules 13a tend to align vertically along the electric field when the sufficient voltage is applied between the electrodes 12, 15. However, as shown in FIG. 2B, since the liquid crystal molecules 13a located in vicinity of the substrates 11, 14 still remain in almost parallel with the substrates 11, 14 by the anchoring effect, the liquid crystal molecules 13a located near the substrates 11, 14 have a smaller inclination to the substrates 11, 14 due to the influence of such effect. In this manner, if portions in which the direction of the liquid crystal molecules is not changed even when the voltage is applied are present, the retardation is generated to enhance the luminance in the dark display and thus the contrast characteristic is degraded.
In order to avoid such disadvantages, it has been proposed to design the phase plate while taking account of the residual retardation at the time of voltage application (Y. Itoh, N. Kimura, S. Mizushima, Y. Ishii and M. Hijikigawa, AM-LCD 2000 digest, p. 243 (2000): (Non-Patent Literature 1)). However, even if such design is applied, it is difficult to eliminate the retardation over the full wavelength range.
FIG. 3 is a schematic view showing an example of another liquid crystal display device in the prior art. The liquid crystal display device of this type is called the VA liquid crystal display device because the vertical alignment (VA) liquid crystal is employed. The VA liquid crystal display device is set forth in U.S. Pat. No. 4,701,028 specification: (Patent Literature 4), for example.
In the VA liquid crystal display device, a vertical alignment liquid crystal 23 is sealed between a first substrate 21 and a second substrate 24. A reflective electrode 22 is formed on a surface of the first substrate 21 on the liquid crystal side. A transparent electrode 25 is formed on a surface of the second substrate 24 on the liquid crystal side. Surfaces of the reflective electrode 22 and the transparent electrode 25 are covered with a vertical alignment film (not shown).
Also, a phase plate (¼ wavelength plate) 26 is arranged on the second substrate 24, and a polarization plate 27 is arranged on the phase plate 26.
In the VA liquid crystal display device constructed in this manner, as shown in FIG. 3, liquid crystal molecules 23a are aligned in the direction perpendicular to the substrates 21, 24 in the condition that no voltage is applied between the electrodes 22, 25. As a result, the retardation is not generated at the time of dark display, and thus the contrast characteristic can be improved in contrast to the TN liquid crystal display device shown in FIG. 1.
In the reflective liquid crystal display device, it is commonly practiced to avoid extreme change in lightness of the image due to the location from which the panel is viewed, by providing a fine roughness on the surface of the reflective electrode. For example, in Patent Application Publication (KOKAI) Hei 9-292504 (Patent Literature 5), the technology to generate the roughness on the surface of the reflective electrode at random with a high density has been proposed. The purposes of this are that interference of a light due to the repeating pattern of the roughness is prevented by increasing randomness of the roughness to prevent the change in color of the reflected light and that a rate of flat portions is reduced by increasing a density of the roughness to reduce a specular reflection component.
Also, the applicant of this application has proposed a method of forming wrinkle-like fine roughness on the surface of the reflective electrode via simple steps by using the photoresist (Patent Application Publication (KOKAI) 2002-221716 (Patent Literature 6), and Patent Application Publication (KOKAI) 2002-296585 (Patent Literature 7)). In this method, only a surface layer of the photoresist film is cured by irradiating the ultraviolet ray, or the like onto the photoresist film, and then the annealing is applied to the photoresist film. Accordingly, the wrinkle-like fine roughness is formed due to a difference in the thermal deformation characteristic (thermal expansion coefficient or thermal contraction coefficient) between the surface layer and the deep portion of the photoresist film. Then, the reflective electrode having the fine roughness on the surface is formed by forming a metal film made of aluminum, or the like on the photoresist film and patterning this metal film.
In order to implement the bright display in the reflective liquid crystal display device, it is important to optimize the reflecting surface of the reflective electrode. That is, taking account of the situation of practical use, if the reflecting surface of the reflective electrode is formed to reflect mainly the light incident on the panel from the upper side in the normal direction of the panel, a utilization efficiency of light can be improved and thus the bright image can be obtained.
For example, as shown in FIG. 4A, in Patent Application Publication (KOKAI) Hei 9-127501 (Patent Literature 8), the reflective liquid crystal display device in which a reflective electrode 32 having a blaze (sawtooth)-like reflecting surface and a light scattering body 36 are combined together has been proposed. In this reflective liquid crystal display device, the panel is constructed by sealing a liquid crystal 33 between a pair of substrates 31, 34 and the light scattering body 36 is arranged on the substrate 34 side.
However, in the method set forth in Patent Application Publication (KOKAI) Hei 9-127501, the production process is very complicated because the blaze-like reflecting surface is formed by the transfer by virtue of the die. In addition, since the blaze-like reflecting surface is formed as a specular surface and is combined with the scattering plate, the light is scattered in both cases when the light is incident on the panel and when the light is emitted from the panel, and therefore the image becomes obscured.
In C. J. Wen, D. L. Ting, C. Y. Chen, L. S. Chuang and C. C. Chang, SID'00 digest of technical papers, p. 526 (2000) (Non-Patent Literature 2), the reflective liquid crystal display device in which the roughness is provided on the blaze-like reflecting surface of the reflective electrode, as shown in FIG. 4B, by using the photolithography method has been proposed. According to this method, the roughness is formed on a reflecting surface of a reflective electrode 42 by applying plural times the photolithography step repeatedly. Then, a liquid crystal 43 is sealed between a substrate 41 and a substrate 44.
In Patent Application Publication (KOKAI) Sho 57-102680 (Patent Literature 9), the technology to limit an average inclination angle of the roughness so as to converge the scattered light within a predetermined limited range and thus get the bright display has been proposed. In addition, in Japanese Patent No. 3187369 Specification (Patent Literature 10), the reflective electrode in which the existence rate of the inclination angle in the particular range is increased as the inclination angle is increased has been proposed. Also, the reflective liquid crystal display device in which the uniform brightness can be obtained in the effective viewing angle is disclosed.
(Patent Literature 1)
Patent Application Publication (KOKAI) Sho 55-48733
(Patent Literature 2)
Patent Application Publication (KOKAI) Hei 2-236523
(Patent Literature 3)
Patent Application Publication (KOKAI) Hei 6-167708
(Patent Literature 4)
U.S. Pat. No. 4,701,028
(Patent Literature 5)
Patent Application Publication (KOKAI) Hei 9-292504
(Patent Literature 6)
Patent Application Publication (KOKAI) 2002-221716
(Patent Literature 7)
Patent Application Publication (KOKAI) 2002-296585
(Patent Literature 8)
Patent Application Publication (KOKAI) Hei 9-127501
(Patent Literature 9)
Patent Application Publication (KOKAI) Sho 57-102680
(Patent Literature 10)
Japanese Patent No. 3187369 Specification
(Non-Patent Literature 1)
Y. Itoh, N. Kimura, S. Mizushima, Y. Ishii and M. Hijikigawa, AM-LCD 2000 digest, p. 243 (2000)
(Non-Patent Literature 2)
C. J. Wen, D. L. Ting, C. Y. Chen, L. S. Chuang and C. C. Chang, SID' 00 digest of technical papers, p. 526 (2000)
However, according to above all methods in the prior art, there exist the problems that the step of shaping a reflecting surface of the reflective electrode into a predetermined shape becomes complicated and thus an increase in production cost is brought about.